The present invention relates to a method for generating a tripping signal as a function of the magnitude and the duration of an overcurrent, in which a rectified measurement variable derived from the overcurrent is sampled and the sampled measurement values are converted into corresponding digital values, the digital values are then weighted according to a predetermined function and summed accordingly and a tripping signal is formed if the sum value is above a predetermined reference value and the summation is cancelled before the tripping signal is formed, if the overcurrent drops below a given threshold for a period of time.
A known monitoring equipment which is described in DE-OS No. 29 50 031 (see U.S. Pat. No. 4,219,860) operates in accordance with such a method, since in this known monitoring equipment a measurement value obtained by rectification from the current to be monitored is sampled by means of the sampling circuit and the sampled values so obtained are converted into corresponding digital values in an A/D converter. The digital values are weighted in a function generator and added up after being weighted in an adding circuit. If the state of the adding circuit reaches a predetermined reference value, a tripping signal is generated. In the known monitoring equipment, the A/D converter is followed by a circuit module which checks continuously whether the current to be monitored has overcurrent values. As long as this is the case, a clock-controlled counter following the circuit module is reset continuously. If the resetting signal of the circuit module fails to arrive, the clock-controlled counter runs up to a predetermined counter reading and then resets the adding circuit. So that resetting of the adding circuit does not occur when, due to the waveform of the rectified measurement value, the instantaneous values drop below a value characterizing an overcurrent value, the predetermined counter must be designed so that in the known monitoring device this counter reading can be reached only after a time which is longer than the duration of a half-wave of the current to be monitored. This can lead to a situation whereby, in isolated cases, tripping takes place needlessly. In any case, the known monitoring equipment for currents of 50 or 60 Hz to be monitored, which operates in accordance with the described method, requires counters which are set differently with respect to the critical counter reading.